The first thing you should do is modify your post and remove your registration key before it gets harvested and used on another system, I don’t know what would happen if multiple copies of your key were found to be in use.

Looks like you purchased avast Pro from an avast reseller, your trial version should still have updated through the avast.com domain.

I would suggest that you download the latest Pro version http://www.avast.com/eng/download-avast-professional.html, select the Language from the list, save it to your HDD somewhere you can find it later.

Then I would suggest a clean reinstall.
Download the avast! Uninstall Utility, find it here and save it to your HDD.
Now uninstall, reboot, run the Uninstall Utility, reboot install the latest version, reboot.

Once installed you can enter your registration key.